A British amateur chemist has worked out how the ancient Romans dyed the togas of emperors this deep colour thanks to a bacterium found in cockles from the supermarket Tesco.

The hue had special significance as the colour of imperial power. Cleopatra also had the sails on her ship dyed the same colour.

The recipe for the dye had been kept a craft secret, even in ancient Egypt and Rome. There are few references to the dying process in the historical literature.

Green to purple

Modern chemistry can make every shade of every colour, but retired engineer John Edmonds is interested in how the ancients managed to make dyes from natural materials.

...and of Egypt's pharoahs

He explained to the British Association science festival in Salford, Greater Manchester, how he rediscovered the secret of imperial purple after studying the fermentation process of indigo pigments from the woad plant.

With help of researchers in Reading and from Israel he has been able to establish the vital role played by a bacterium in chemically reducing (the addition of electrons) the ancient pigments so that they will dissolve in a dye solution.

The pigment for imperial purple was derived from Murex molluscs, a form of shellfish. So, Mr Edmunds reasoned that he could try to use the related common cockle.

He bought a jar of them from Tesco. "Having removed the vinegar, I placed several of the cockles with some of the purple pigment in a vat consisting of a 2 lb jam jar."

Modern jeans

The cockles are thought to harbour a bacterium that is crucial in reducing the dye. Wood ash was added to the vat to ensure the mixture did not turn acidic.

The mixture was then kept at 50 Celsius for about 10 days.

Wool dipped in the pigment turned green at first but, eventually, in contact with light, it turned purple.

The recreation of the old dying method might have implications for present-day practice.

Currently, tonnes of chemicals are needed to reduce the dye for denim blue jeans, resulting in large quantities of sulphur waste.

Mr Edmonds said: "University of Reading scientists are trying to understand how the bacterium reduces indigo in order to develop a clean biotechnology to replace the chemical process for indigo reduction in the future."